That's kind of the plan, I guess (the "wait and see" method). The concrete step outside does need to be refinished and sealed (some exposed aggregate and the seam that needs patching).
However, how will I know if it's working? Wouldn't I have to pull everything off and check the condition of the concrete? That would be a hassle, but not as bad as having to pull drywall down I guess.
Also ToolGuy, the plastic isn't about the expense or time, it's about whether adding that vapor barrier would trap moisture between itself and the foam. I'm basing this assumption on other things I've read about having 2 vapor barriers on the interior side...
Anyway, thanks for the continued help and ideas. I'm going to spend the rest of the winter getting the rest of the basement insulated in a similar way. Luckily, this is the only place where water was an issue...the rest of the basement is very dry.